Image forming apparatuses read images from documents, form developer images corresponding to the images read, on paper sheets, and fix the developer images on the paper sheets by means of a fixing apparatus.
In the fixing apparatus, a paper sheet is held between the heat roller and the press roller, and heat and pressure are applied to the paper sheet. The developer image on the paper sheet is thereby fixed.
A center coil and side coils are provided within or outside the heat roller. These coils generate high-frequency magnetic fields when a high-frequency current is supplied to them. From the high-frequency magnetic fields there are generated eddy currents. The eddy currents turn into Joule heat. The Joule heat heats the heat roller.
The center coil performs induction heating on that part of the heat roller that is almost middle in the axial direction of the heat roller (i.e., the direction at right angles to the direction in which the heat roller rotates). The side coils perform induction heating at one end of the heat roller and the other end thereof, respectively.
In the fixing apparatus, the center coil and the side coils are alternately driven. The temperatures of the middle part and end parts of the heat roller are detected. The output of the center coil and the output of the side coils are controlled by means of pulse-width modulation, so that the temperatures detected may remain at a preset value.
It is too much for the control means, such as a CPU, to drive the center coil and the side coils alternately and to control the temperature of the heat roller by performing pulse-width modulation.